It's better to exhaust. The reason is if you blow in, you create turbulence that mixes your cool and warm air, and a mix exits the other window, limiting effectiveness. If you exhaust, the incoming air from opposite window is less turbulent, doing a better job of air exchange,

The next part is that fan is probably 100 watts of power and if you put that fan blowing in, that 100 watts goes on to the room. Put the fan in the room, close both windows and run it, the room will heat slowly.

All electric devices are 100% efficient at taking the used wattage as measured coming in, and turning it into heat. Incandescent light bulb, fan motor, space heater, mini fridge.... If the device is in a sealed featureless room, it matters not what it does, the power used is turned into heat that is dissipated into the room.

Engineer here. You want to create air flow through the space, so place your window fan blowing outward to pull air into the space. If your windows oppose one another, or are on the same wall at opposite ends of the wall, this works out great. If windows are adjacent then it's not very effective. In the case of adjacent windows, go to another nearby room and open a window there. Prop open your doors between the two rooms and pull air through both. With a small enough house, you can use this to cool the entire house down.